1. Field of the Invention
The present invention relates to an optical transmission circuit, and more particularly, to a circuit system of a laser diode drive circuit for an optical communication system.
2. Description of the Related Art
Conventionally, there has been known an optical transmission circuit which drives a light emitting device by a differential modulator (for example, see JP 2004-193489 A).
FIG. 5 is a circuit diagram illustrating an example of such a conventional optical transmission circuit 20. As illustrated in FIG. 5, the optical transmission circuit 20 includes a laser diode 800, a modulator 900, and a current source 101. The modulator 900 supplies a differential modulation current to the laser diode 800 through transmission lines 301, 302, 303, and 304, and AC coupling capacitors 701 and 702. The current source 101 supplies a bias current to the laser diode 800 through inductors 201 and 202.
In the conventional optical transmission circuit 20 illustrated in FIG. 5, the inductors 201 and 202 provided at a cathode terminal and an anode terminal of the laser diode 800, respectively, are formed to have a high impedance with respect to an AC signal such as a modulation current. Accordingly, leakage of a modulation current to the current source 101, a ground line, and a power supply line is suppressed by the inductors 201 and 202, whereby the differential modulation current efficiently flows through the laser diode 800.
As a specific device of the current source 101, a bipolar transistor or a field effect transistor is used. As the inductors 201 and 202, a chip inductor or a chip bead of several μH to several tens μH is used in some cases by taking a mounting area or permissible rated current into consideration.
Characteristic impedances of the transmission lines 301, 302, 303, and 304 are set to approximately 25Ω to 50Ω so as to achieve matching with an output impedance of the modulator 900.
FIG. 6 is a graph illustrating an example of a forward voltage and a light output intensity with respect to a forward current of the laser diode 800. As illustrated in FIG. 6, in the laser diode 800, the light output intensity and the forward voltage increase as the forward current increases. A bias current of about several tens mA is supplied when the laser diode 800 is operated, and on this occasion, an impedance of the laser diode 800, which is obtained by a slope of the forward current and a slope of the forward voltage, is approximately several Ω to a dozen or so of Ω, which is smaller compared with the transmission lines 301, 302, 303, and 304.
For this reason, impedance mismatch occurs at a connection point between the transmission line 303 and the laser diode 800 and a connection point between the transmission line 304 and the laser diode 800, and hence, in some cases, a part of a high-frequency signal is not transmitted to the laser diode 800 to return to the modulator 900 as a reflected wave. In addition, though slightly, impedance mismatch actually exists at a connection point between the modulator 900 and the transmission line 301 and a connection point between the modulator 900 and the transmission line 302, and hence, in some cases, a part of the reflected wave returning to the modulator 900 is further reflected at the connection point between the modulator 900 and the transmission line 301 and the connection point between the modulator 900 and the transmission line 302. In other words, reflection is generated at both ends of the transmission line, which are impedance mismatch points, and accordingly, a resonance phenomenon occurs in some cases. A multiple reflected wave which generates such a resonance phenomenon is superimposed on an electric signal supplied to the laser diode 800 as high-frequency noise, whereby the high-frequency noise is also superimposed on an optical signal generated by the laser diode 800.
In order to solve a problem of the impedance mismatch, there is a technique of reducing the characteristic impedances of the transmission lines 301, 302, 303, and 304 and the output impedance of the modulator 900 to approximately several Ω to a dozen or so of Ω, which is the impedance of the laser diode 800. However, in order to ensure a voltage amplitude required to drive the laser diode 800 in the state in which the output impedance of the modulator 900 is reduced to approximately several Ω to a dozen or so of Ω, there arises another problem that power consumption of the modulator 900 increases. Hereinafter, the reason why the power consumption of the modulator 900 increases is described.
FIG. 7 is a circuit diagram illustrating a typical circuit configuration example of the modulator 900. The modulator 900 illustrated in FIG. 7 includes a current source 931, transistors 921 and 922 which have emitter terminals connected to each other to form a differential current switch, and output resistors 911 and 912.
As illustrated in FIG. 7, when a differential voltage signal is applied to base terminals of the transistors 921 and 922, one of the transistors 921 and 922 which form the differential current switch is in an on state, and another thereof is in an off state, whereby a current supplied from the current source 931 selectively flows through any one of the output resistor 911 and the output resistor 912. Further, a voltage amplitude for driving the laser diode 800 is determined by a product of resistance values of the output resistors 911 and 912 and values of currents flowing through the output resistors 911 and 912 (current amount of the current source 931).
Here, in order to reduce the output impedance of the modulator 900 to approximately several Ω to a dozen or so of Ω which is an impedance of the laser diode 800, the resistance values of the output resistors 911 and 912 each need to be set to approximately several Ω to a dozen or so of Ω. However, the voltage amplitude required for driving the laser diode 800 cannot be obtained by such low output resistance values, and thus the current of the current source 931 needs to be increased so as to compensate for a decrease in output resistance.
As described above, when the output impedance of the modulator 900 is reduced to approximately several Ω to a dozen or so of Ω which is the impedance of the laser diode 800, impedance mismatch is resolved. However, there arises another problem that power consumption of the modulator 900 increases. In other words, it is difficult to make both an increase in quality of the optical signal waveform and a decrease in power consumption coexist by the technique of matching the output impedance of the modulator 900 to the impedance of the laser diode 800.